1 Network Working Group R. Arends 2 Request for Comments: 4033 Telematica Instituut 3 Obsoletes: 2535, 3008, 3090, 3445, 3655, 3658, R. Austein 4 3755, 3757, 3845 ISC
The IETF is responsible for the creation and maintenance of the DNS RFCs. The ICANN DNS RFC annotation project provides a forum for collecting community annotations on these RFCs as an aid to understanding for implementers and any interested parties. The annotations displayed here are not the result of the IETF consensus process.
This RFC is included in the DNS RFCs annotation project whose home page is here.
This RFC is implemented in BIND 9.18 (all versions).
5 Updates: 1034, 1035, 2136, 2181, 2308, 3225, M. Larson 6 3007, 3597, 3226 VeriSign 7 Category: Standards Track D. Massey 8 Colorado State University 9 S. Rose 10 NIST 11 March 2005 12 13 14 DNS Security Introduction and Requirements 15 16 Status of This Memo 17 18 This document specifies an Internet standards track protocol for the 19 Internet community, and requests discussion and suggestions for 20 improvements. Please refer to the current edition of the "Internet 21 Official Protocol Standards" (STD 1) for the standardization state 22 and status of this protocol. Distribution of this memo is unlimited. 23 24 Copyright Notice 25 26 Copyright (C) The Internet Society (2005). 27 28 Abstract 29 30 The Domain Name System Security Extensions (DNSSEC) add data origin 31 authentication and data integrity to the Domain Name System. This 32 document introduces these extensions and describes their capabilities 33 and limitations. This document also discusses the services that the 34 DNS security extensions do and do not provide. Last, this document 35 describes the interrelationships between the documents that 36 collectively describe DNSSEC. 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 Arends, et al. Standards Track [Page 1] 53 RFC 4033 DNS Security Introduction and Requirements March 2005 54 55 56 Table of Contents 57 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 59 2. Definitions of Important DNSSEC Terms . . . . . . . . . . . 3 60 3. Services Provided by DNS Security . . . . . . . . . . . . . 7 61 3.1. Data Origin Authentication and Data Integrity . . . . 7 62 3.2. Authenticating Name and Type Non-Existence . . . . . . 9 63 4. Services Not Provided by DNS Security . . . . . . . . . . . 9 64 5. Scope of the DNSSEC Document Set and Last Hop Issues . . . . 9 65 6. Resolver Considerations . . . . . . . . . . . . . . . . . . 10 66 7. Stub Resolver Considerations . . . . . . . . . . . . . . . . 11 67 8. Zone Considerations . . . . . . . . . . . . . . . . . . . . 12 68 8.1. TTL Values vs. RRSIG Validity Period . . . . . . . . . 13 69 8.2. New Temporal Dependency Issues for Zones . . . . . . . 13 70 9. Name Server Considerations . . . . . . . . . . . . . . . . . 13 71 10. DNS Security Document Family . . . . . . . . . . . . . . . . 14 72 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 15 73 12. Security Considerations . . . . . . . . . . . . . . . . . . 15 74 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 75 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 76 14.1. Normative References . . . . . . . . . . . . . . . . . 17 77 14.2. Informative References . . . . . . . . . . . . . . . . 18 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 79 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 21 80 81 1. Introduction 82 83 This document introduces the Domain Name System Security Extensions 84 (DNSSEC). This document and its two companion documents ([RFC4034] 85 and [RFC4035]) update, clarify, and refine the security extensions 86 defined in [RFC2535] and its predecessors. These security extensions 87 consist of a set of new resource record types and modifications to 88 the existing DNS protocol ([RFC1035]). The new records and protocol 89 modifications are not fully described in this document, but are 90 described in a family of documents outlined in Section 10. Sections 91 3 and 4 describe the capabilities and limitations of the security 92 extensions in greater detail. Section 5 discusses the scope of the 93 document set. Sections 6, 7, 8, and 9 discuss the effect that these 94 security extensions will have on resolvers, stub resolvers, zones, 95 and name servers. 96 97 This document and its two companions obsolete [RFC2535], [RFC3008], 98 [RFC3090], [RFC3445], [RFC3655], [RFC3658], [RFC3755], [RFC3757], and 99 [RFC3845]. This document set also updates but does not obsolete 100 [RFC1034], [RFC1035], [RFC2136], [RFC2181], [RFC2308], [RFC3225], 101 [RFC3007], [RFC3597], and the portions of [RFC3226] that deal with 102 DNSSEC. 103 104 105 106 107 Arends, et al. Standards Track [Page 2] 108 RFC 4033 DNS Security Introduction and Requirements March 2005 109 110 111 The DNS security extensions provide origin authentication and 112 integrity protection for DNS data, as well as a means of public key 113 distribution. These extensions do not provide confidentiality. 114 115 2. Definitions of Important DNSSEC Terms 116 117 This section defines a number of terms used in this document set. 118 Because this is intended to be useful as a reference while reading 119 the rest of the document set, first-time readers may wish to skim 120 this section quickly, read the rest of this document, and then come 121 back to this section. 122 123 Authentication Chain: An alternating sequence of DNS public key 124 (DNSKEY) RRsets and Delegation Signer (DS) RRsets forms a chain of 125 signed data, with each link in the chain vouching for the next. A 126 DNSKEY RR is used to verify the signature covering a DS RR and 127 allows the DS RR to be authenticated. The DS RR contains a hash 128 of another DNSKEY RR and this new DNSKEY RR is authenticated by 129 matching the hash in the DS RR. This new DNSKEY RR in turn 130 authenticates another DNSKEY RRset and, in turn, some DNSKEY RR in 131 this set may be used to authenticate another DS RR, and so forth 132 until the chain finally ends with a DNSKEY RR whose corresponding 133 private key signs the desired DNS data. For example, the root 134 DNSKEY RRset can be used to authenticate the DS RRset for 135 "example." The "example." DS RRset contains a hash that matches 136 some "example." DNSKEY, and this DNSKEY's corresponding private 137 key signs the "example." DNSKEY RRset. Private key counterparts 138 of the "example." DNSKEY RRset sign data records such as 139 "www.example." and DS RRs for delegations such as 140 "subzone.example." 141 142 Authentication Key: A public key that a security-aware resolver has 143 verified and can therefore use to authenticate data. A 144 security-aware resolver can obtain authentication keys in three 145 ways. First, the resolver is generally configured to know about 146 at least one public key; this configured data is usually either 147 the public key itself or a hash of the public key as found in the 148 DS RR (see "trust anchor"). Second, the resolver may use an 149 authenticated public key to verify a DS RR and the DNSKEY RR to 150 which the DS RR refers. Third, the resolver may be able to 151 determine that a new public key has been signed by the private key 152 corresponding to another public key that the resolver has 153 verified. Note that the resolver must always be guided by local 154 policy when deciding whether to authenticate a new public key, 155 even if the local policy is simply to authenticate any new public 156 key for which the resolver is able verify the signature. 157 158 159 160 161 162 Arends, et al. Standards Track [Page 3] 163 RFC 4033 DNS Security Introduction and Requirements March 2005 164 165 166 Authoritative RRset: Within the context of a particular zone, an 167 RRset is "authoritative" if and only if the owner name of the 168 RRset lies within the subset of the name space that is at or below 169 the zone apex and at or above the cuts that separate the zone from 170 its children, if any. All RRsets at the zone apex are 171 authoritative, except for certain RRsets at this domain name that, 172 if present, belong to this zone's parent. These RRset could 173 include a DS RRset, the NSEC RRset referencing this DS RRset (the 174 "parental NSEC"), and RRSIG RRs associated with these RRsets, all 175 of which are authoritative in the parent zone. Similarly, if this 176 zone contains any delegation points, only the parental NSEC RRset, 177 DS RRsets, and any RRSIG RRs associated with these RRsets are 178 authoritative for this zone. 179 180 Delegation Point: Term used to describe the name at the parental side 181 of a zone cut. That is, the delegation point for "foo.example" 182 would be the foo.example node in the "example" zone (as opposed to 183 the zone apex of the "foo.example" zone). See also zone apex. 184 185 Island of Security: Term used to describe a signed, delegated zone 186 that does not have an authentication chain from its delegating 187 parent. That is, there is no DS RR containing a hash of a DNSKEY 188 RR for the island in its delegating parent zone (see [RFC4034]). 189 An island of security is served by security-aware name servers and 190 may provide authentication chains to any delegated child zones. 191 Responses from an island of security or its descendents can only 192 be authenticated if its authentication keys can be authenticated 193 by some trusted means out of band from the DNS protocol. 194 195 Key Signing Key (KSK): An authentication key that corresponds to a 196 private key used to sign one or more other authentication keys for 197 a given zone. Typically, the private key corresponding to a key 198 signing key will sign a zone signing key, which in turn has a 199 corresponding private key that will sign other zone data. Local 200 policy may require that the zone signing key be changed 201 frequently, while the key signing key may have a longer validity 202 period in order to provide a more stable secure entry point into 203 the zone. Designating an authentication key as a key signing key 204 is purely an operational issue: DNSSEC validation does not 205 distinguish between key signing keys and other DNSSEC 206 authentication keys, and it is possible to use a single key as 207 both a key signing key and a zone signing key. Key signing keys 208 are discussed in more detail in [RFC3757]. Also see zone signing 209 key. 210 211 212 213 214 215 216 217 Arends, et al. Standards Track [Page 4] 218 RFC 4033 DNS Security Introduction and Requirements March 2005 219 220 221 Non-Validating Security-Aware Stub Resolver: A security-aware stub 222 resolver that trusts one or more security-aware recursive name 223 servers to perform most of the tasks discussed in this document 224 set on its behalf. In particular, a non-validating security-aware 225 stub resolver is an entity that sends DNS queries, receives DNS 226 responses, and is capable of establishing an appropriately secured 227 channel to a security-aware recursive name server that will 228 provide these services on behalf of the security-aware stub 229 resolver. See also security-aware stub resolver, validating 230 security-aware stub resolver. 231 232 Non-Validating Stub Resolver: A less tedious term for a 233 non-validating security-aware stub resolver. 234 235 Security-Aware Name Server: An entity acting in the role of a name 236 server (defined in section 2.4 of [RFC1034]) that understands the 237 DNS security extensions defined in this document set. In 238 particular, a security-aware name server is an entity that 239 receives DNS queries, sends DNS responses, supports the EDNS0 240 ([RFC2671]) message size extension and the DO bit ([RFC3225]), and 241 supports the RR types and message header bits defined in this 242 document set. 243 244 Security-Aware Recursive Name Server: An entity that acts in both the 245 security-aware name server and security-aware resolver roles. A 246 more cumbersome but equivalent phrase would be "a security-aware 247 name server that offers recursive service". 248 249 Security-Aware Resolver: An entity acting in the role of a resolver 250 (defined in section 2.4 of [RFC1034]) that understands the DNS 251 security extensions defined in this document set. In particular, 252 a security-aware resolver is an entity that sends DNS queries, 253 receives DNS responses, supports the EDNS0 ([RFC2671]) message 254 size extension and the DO bit ([RFC3225]), and is capable of using 255 the RR types and message header bits defined in this document set 256 to provide DNSSEC services. 257 258 Security-Aware Stub Resolver: An entity acting in the role of a stub 259 resolver (defined in section 5.3.1 of [RFC1034]) that has enough 260 of an understanding the DNS security extensions defined in this 261 document set to provide additional services not available from a 262 security-oblivious stub resolver. Security-aware stub resolvers 263 may be either "validating" or "non-validating", depending on 264 whether the stub resolver attempts to verify DNSSEC signatures on 265 its own or trusts a friendly security-aware name server to do so. 266 See also validating stub resolver, non-validating stub resolver. 267 268 269 270 271 272 Arends, et al. Standards Track [Page 5] 273 RFC 4033 DNS Security Introduction and Requirements March 2005 274 275 276 Security-Oblivious <anything>: An <anything> that is not 277 "security-aware". 278 279 Signed Zone: A zone whose RRsets are signed and that contains 280 properly constructed DNSKEY, Resource Record Signature (RRSIG), 281 Next Secure (NSEC), and (optionally) DS records. 282 283 Trust Anchor: A configured DNSKEY RR or DS RR hash of a DNSKEY RR. A 284 validating security-aware resolver uses this public key or hash as 285 a starting point for building the authentication chain to a signed 286 DNS response. In general, a validating resolver will have to 287 obtain the initial values of its trust anchors via some secure or 288 trusted means outside the DNS protocol. Presence of a trust 289 anchor also implies that the resolver should expect the zone to 290 which the trust anchor points to be signed. 291 292 Unsigned Zone: A zone that is not signed. 293 294 Validating Security-Aware Stub Resolver: A security-aware resolver 295 that sends queries in recursive mode but that performs signature 296 validation on its own rather than just blindly trusting an 297 upstream security-aware recursive name server. See also 298 security-aware stub resolver, non-validating security-aware stub 299 resolver. 300 301 Validating Stub Resolver: A less tedious term for a validating 302 security-aware stub resolver. 303 304 Zone Apex: Term used to describe the name at the child's side of a 305 zone cut. See also delegation point. 306 307 Zone Signing Key (ZSK): An authentication key that corresponds to a 308 private key used to sign a zone. Typically, a zone signing key 309 will be part of the same DNSKEY RRset as the key signing key whose 310 corresponding private key signs this DNSKEY RRset, but the zone 311 signing key is used for a slightly different purpose and may 312 differ from the key signing key in other ways, such as validity 313 lifetime. Designating an authentication key as a zone signing key 314 is purely an operational issue; DNSSEC validation does not 315 distinguish between zone signing keys and other DNSSEC 316 authentication keys, and it is possible to use a single key as 317 both a key signing key and a zone signing key. See also key 318 signing key. 319 320 321 322 323 324 325 326 327 Arends, et al. Standards Track [Page 6] 328 RFC 4033 DNS Security Introduction and Requirements March 2005 329 330 331 3. Services Provided by DNS Security 332 333 The Domain Name System (DNS) security extensions provide origin 334 authentication and integrity assurance services for DNS data, 335 including mechanisms for authenticated denial of existence of DNS 336 data. These mechanisms are described below. 337 338 These mechanisms require changes to the DNS protocol. DNSSEC adds 339 four new resource record types: Resource Record Signature (RRSIG), 340 DNS Public Key (DNSKEY), Delegation Signer (DS), and Next Secure 341 (NSEC). It also adds two new message header bits: Checking Disabled 342 (CD) and Authenticated Data (AD). In order to support the larger DNS 343 message sizes that result from adding the DNSSEC RRs, DNSSEC also 344 requires EDNS0 support ([RFC2671]). Finally, DNSSEC requires support 345 for the DNSSEC OK (DO) EDNS header bit ([RFC3225]) so that a 346 security-aware resolver can indicate in its queries that it wishes to 347 receive DNSSEC RRs in response messages. 348 349 These services protect against most of the threats to the Domain Name 350 System described in [RFC3833]. Please see Section 12 for a 351 discussion of the limitations of these extensions. 352 353 3.1. Data Origin Authentication and Data Integrity 354 355 DNSSEC provides authentication by associating cryptographically 356 generated digital signatures with DNS RRsets. These digital 357 signatures are stored in a new resource record, the RRSIG record. 358 Typically, there will be a single private key that signs a zone's 359 data, but multiple keys are possible. For example, there may be keys 360 for each of several different digital signature algorithms. If a 361 security-aware resolver reliably learns a zone's public key, it can 362 authenticate that zone's signed data. An important DNSSEC concept is 363 that the key that signs a zone's data is associated with the zone 364 itself and not with the zone's authoritative name servers. (Public 365 keys for DNS transaction authentication mechanisms may also appear in 366 zones, as described in [RFC2931], but DNSSEC itself is concerned with 367 object security of DNS data, not channel security of DNS 368 transactions. The keys associated with transaction security may be 369 stored in different RR types. See [RFC3755] for details.) 370 371 A security-aware resolver can learn a zone's public key either by 372 having a trust anchor configured into the resolver or by normal DNS 373 resolution. To allow the latter, public keys are stored in a new 374 type of resource record, the DNSKEY RR. Note that the private keys 375 used to sign zone data must be kept secure and should be stored 376 offline when practical. To discover a public key reliably via DNS 377 resolution, the target key itself has to be signed by either a 378 configured authentication key or another key that has been 379 380 381 382 Arends, et al. Standards Track [Page 7] 383 RFC 4033 DNS Security Introduction and Requirements March 2005 384 385 386 authenticated previously. Security-aware resolvers authenticate zone 387 information by forming an authentication chain from a newly learned 388 public key back to a previously known authentication public key, 389 which in turn either has been configured into the resolver or must 390 have been learned and verified previously. Therefore, the resolver 391 must be configured with at least one trust anchor. 392 393 If the configured trust anchor is a zone signing key, then it will 394 authenticate the associated zone; if the configured key is a key 395 signing key, it will authenticate a zone signing key. If the 396 configured trust anchor is the hash of a key rather than the key 397 itself, the resolver may have to obtain the key via a DNS query. To 398 help security-aware resolvers establish this authentication chain, 399 security-aware name servers attempt to send the signature(s) needed 400 to authenticate a zone's public key(s) in the DNS reply message along 401 with the public key itself, provided that there is space available in 402 the message. 403 404 The Delegation Signer (DS) RR type simplifies some of the 405 administrative tasks involved in signing delegations across 406 organizational boundaries. The DS RRset resides at a delegation 407 point in a parent zone and indicates the public key(s) corresponding 408 to the private key(s) used to self-sign the DNSKEY RRset at the 409 delegated child zone's apex. The administrator of the child zone, in 410 turn, uses the private key(s) corresponding to one or more of the 411 public keys in this DNSKEY RRset to sign the child zone's data. The 412 typical authentication chain is therefore 413 DNSKEY->[DS->DNSKEY]*->RRset, where "*" denotes zero or more 414 DS->DNSKEY subchains. DNSSEC permits more complex authentication 415 chains, such as additional layers of DNSKEY RRs signing other DNSKEY 416 RRs within a zone. 417 418 A security-aware resolver normally constructs this authentication 419 chain from the root of the DNS hierarchy down to the leaf zones based 420 on configured knowledge of the public key for the root. Local 421 policy, however, may also allow a security-aware resolver to use one 422 or more configured public keys (or hashes of public keys) other than 423 the root public key, may not provide configured knowledge of the root 424 public key, or may prevent the resolver from using particular public 425 keys for arbitrary reasons, even if those public keys are properly 426 signed with verifiable signatures. DNSSEC provides mechanisms by 427 which a security-aware resolver can determine whether an RRset's 428 signature is "valid" within the meaning of DNSSEC. In the final 429 analysis, however, authenticating both DNS keys and data is a matter 430 of local policy, which may extend or even override the protocol 431 extensions defined in this document set. See Section 5 for further 432 discussion. 433 434 435 436 437 Arends, et al. Standards Track [Page 8] 438 RFC 4033 DNS Security Introduction and Requirements March 2005 439 440 441 3.2. Authenticating Name and Type Non-Existence 442 443 The security mechanism described in Section 3.1 only provides a way 444 to sign existing RRsets in a zone. The problem of providing negative 445 responses with the same level of authentication and integrity 446 requires the use of another new resource record type, the NSEC 447 record. The NSEC record allows a security-aware resolver to 448 authenticate a negative reply for either name or type non-existence 449 with the same mechanisms used to authenticate other DNS replies. Use 450 of NSEC records requires a canonical representation and ordering for 451 domain names in zones. Chains of NSEC records explicitly describe 452 the gaps, or "empty space", between domain names in a zone and list 453 the types of RRsets present at existing names. Each NSEC record is 454 signed and authenticated using the mechanisms described in Section 455 3.1. 456 457 4. Services Not Provided by DNS Security 458 459 DNS was originally designed with the assumptions that the DNS will 460 return the same answer to any given query regardless of who may have 461 issued the query, and that all data in the DNS is thus visible. 462 Accordingly, DNSSEC is not designed to provide confidentiality, 463 access control lists, or other means of differentiating between 464 inquirers. 465 466 DNSSEC provides no protection against denial of service attacks. 467 Security-aware resolvers and security-aware name servers are 468 vulnerable to an additional class of denial of service attacks based 469 on cryptographic operations. Please see Section 12 for details. 470 471 The DNS security extensions provide data and origin authentication 472 for DNS data. The mechanisms outlined above are not designed to 473 protect operations such as zone transfers and dynamic update 474 ([RFC2136], [RFC3007]). Message authentication schemes described in 475 [RFC2845] and [RFC2931] address security operations that pertain to 476 these transactions. 477 478 5. Scope of the DNSSEC Document Set and Last Hop Issues 479 480 The specification in this document set defines the behavior for zone 481 signers and security-aware name servers and resolvers in such a way 482 that the validating entities can unambiguously determine the state of 483 the data. 484 485 A validating resolver can determine the following 4 states: 486 487 Secure: The validating resolver has a trust anchor, has a chain of 488 trust, and is able to verify all the signatures in the response. 489 490 491 492 Arends, et al. Standards Track [Page 9] 493 RFC 4033 DNS Security Introduction and Requirements March 2005 494 495 496 Insecure: The validating resolver has a trust anchor, a chain of 497 trust, and, at some delegation point, signed proof of the 498 non-existence of a DS record. This indicates that subsequent 499 branches in the tree are provably insecure. A validating resolver 500 may have a local policy to mark parts of the domain space as 501 insecure. 502 503 Bogus: The validating resolver has a trust anchor and a secure 504 delegation indicating that subsidiary data is signed, but the 505 response fails to validate for some reason: missing signatures, 506 expired signatures, signatures with unsupported algorithms, data 507 missing that the relevant NSEC RR says should be present, and so 508 forth. 509 510 Indeterminate: There is no trust anchor that would indicate that a 511 specific portion of the tree is secure. This is the default 512 operation mode. 513 514 This specification only defines how security-aware name servers can 515 signal non-validating stub resolvers that data was found to be bogus 516 (using RCODE=2, "Server Failure"; see [RFC4035]). 517 518 There is a mechanism for security-aware name servers to signal 519 security-aware stub resolvers that data was found to be secure (using 520 the AD bit; see [RFC4035]). 521 522 This specification does not define a format for communicating why 523 responses were found to be bogus or marked as insecure. The current 524 signaling mechanism does not distinguish between indeterminate and 525 insecure states. 526 527 A method for signaling advanced error codes and policy between a 528 security-aware stub resolver and security-aware recursive nameservers 529 is a topic for future work, as is the interface between a security- 530 aware resolver and the applications that use it. Note, however, that 531 the lack of the specification of such communication does not prohibit 532 deployment of signed zones or the deployment of security aware 533 recursive name servers that prohibit propagation of bogus data to the 534 applications. 535 536 6. Resolver Considerations 537 538 A security-aware resolver has to be able to perform cryptographic 539 functions necessary to verify digital signatures using at least the 540 mandatory-to-implement algorithm(s). Security-aware resolvers must 541 also be capable of forming an authentication chain from a newly 542 learned zone back to an authentication key, as described above. This 543 process might require additional queries to intermediate DNS zones to 544 545 546 547 Arends, et al. Standards Track [Page 10] 548 RFC 4033 DNS Security Introduction and Requirements March 2005 549 550 551 obtain necessary DNSKEY, DS, and RRSIG records. A security-aware 552 resolver should be configured with at least one trust anchor as the 553 starting point from which it will attempt to establish authentication 554 chains. 555 556 If a security-aware resolver is separated from the relevant 557 authoritative name servers by a recursive name server or by any sort 558 of intermediary device that acts as a proxy for DNS, and if the 559 recursive name server or intermediary device is not security-aware, 560 the security-aware resolver may not be capable of operating in a 561 secure mode. For example, if a security-aware resolver's packets are 562 routed through a network address translation (NAT) device that 563 includes a DNS proxy that is not security-aware, the security-aware 564 resolver may find it difficult or impossible to obtain or validate 565 signed DNS data. The security-aware resolver may have a particularly 566 difficult time obtaining DS RRs in such a case, as DS RRs do not 567 follow the usual DNS rules for ownership of RRs at zone cuts. Note 568 that this problem is not specific to NATs: any security-oblivious DNS 569 software of any kind between the security-aware resolver and the 570 authoritative name servers will interfere with DNSSEC. 571 572 If a security-aware resolver must rely on an unsigned zone or a name 573 server that is not security aware, the resolver may not be able to 574 validate DNS responses and will need a local policy on whether to 575 accept unverified responses. 576 577 A security-aware resolver should take a signature's validation period 578 into consideration when determining the TTL of data in its cache, to 579 avoid caching signed data beyond the validity period of the 580 signature. However, it should also allow for the possibility that 581 the security-aware resolver's own clock is wrong. Thus, a 582 security-aware resolver that is part of a security-aware recursive 583 name server will have to pay careful attention to the DNSSEC 584 "checking disabled" (CD) bit ([RFC4034]). This is in order to avoid 585 blocking valid signatures from getting through to other 586 security-aware resolvers that are clients of this recursive name 587 server. See [RFC4035] for how a secure recursive server handles 588 queries with the CD bit set. 589 590 7. Stub Resolver Considerations 591 592 Although not strictly required to do so by the protocol, most DNS 593 queries originate from stub resolvers. Stub resolvers, by 594 definition, are minimal DNS resolvers that use recursive query mode 595 to offload most of the work of DNS resolution to a recursive name 596 server. Given the widespread use of stub resolvers, the DNSSEC 597 598 599 600 601 602 Arends, et al. Standards Track [Page 11] 603 RFC 4033 DNS Security Introduction and Requirements March 2005 604 605 606 architecture has to take stub resolvers into account, but the 607 security features needed in a stub resolver differ in some respects 608 from those needed in a security-aware iterative resolver. 609 610 Even a security-oblivious stub resolver may benefit from DNSSEC if 611 the recursive name servers it uses are security-aware, but for the 612 stub resolver to place any real reliance on DNSSEC services, the stub 613 resolver must trust both the recursive name servers in question and 614 the communication channels between itself and those name servers. 615 The first of these issues is a local policy issue: in essence, a 616 security-oblivious stub resolver has no choice but to place itself at 617 the mercy of the recursive name servers that it uses, as it does not 618 perform DNSSEC validity checks on its own. The second issue requires 619 some kind of channel security mechanism; proper use of DNS 620 transaction authentication mechanisms such as SIG(0) ([RFC2931]) or 621 TSIG ([RFC2845]) would suffice, as would appropriate use of IPsec. 622 Particular implementations may have other choices available, such as 623 operating system specific interprocess communication mechanisms. 624 Confidentiality is not needed for this channel, but data integrity 625 and message authentication are. 626 627 A security-aware stub resolver that does trust both its recursive 628 name servers and its communication channel to them may choose to 629 examine the setting of the Authenticated Data (AD) bit in the message 630 header of the response messages it receives. The stub resolver can 631 use this flag bit as a hint to find out whether the recursive name 632 server was able to validate signatures for all of the data in the 633 Answer and Authority sections of the response. 634 635 There is one more step that a security-aware stub resolver can take 636 if, for whatever reason, it is not able to establish a useful trust 637 relationship with the recursive name servers that it uses: it can 638 perform its own signature validation by setting the Checking Disabled 639 (CD) bit in its query messages. A validating stub resolver is thus 640 able to treat the DNSSEC signatures as trust relationships between 641 the zone administrators and the stub resolver itself. 642 643 8. Zone Considerations 644 645 There are several differences between signed and unsigned zones. A 646 signed zone will contain additional security-related records (RRSIG, 647 DNSKEY, DS, and NSEC records). RRSIG and NSEC records may be 648 generated by a signing process prior to serving the zone. The RRSIG 649 records that accompany zone data have defined inception and 650 expiration times that establish a validity period for the signatures 651 and the zone data the signatures cover. 652 653 654 655 656 657 Arends, et al. Standards Track [Page 12] 658 RFC 4033 DNS Security Introduction and Requirements March 2005 659 660 661 8.1. TTL Values vs. RRSIG Validity Period 662 663 It is important to note the distinction between a RRset's TTL value 664 and the signature validity period specified by the RRSIG RR covering 665 that RRset. DNSSEC does not change the definition or function of the 666 TTL value, which is intended to maintain database coherency in 667 caches. A caching resolver purges RRsets from its cache no later 668 than the end of the time period specified by the TTL fields of those 669 RRsets, regardless of whether the resolver is security-aware. 670 671 The inception and expiration fields in the RRSIG RR ([RFC4034]), on 672 the other hand, specify the time period during which the signature 673 can be used to validate the covered RRset. The signatures associated 674 with signed zone data are only valid for the time period specified by 675 these fields in the RRSIG RRs in question. TTL values cannot extend 676 the validity period of signed RRsets in a resolver's cache, but the 677 resolver may use the time remaining before expiration of the 678 signature validity period of a signed RRset as an upper bound for the 679 TTL of the signed RRset and its associated RRSIG RR in the resolver's 680 cache. 681 682 8.2. New Temporal Dependency Issues for Zones 683 684 Information in a signed zone has a temporal dependency that did not 685 exist in the original DNS protocol. A signed zone requires regular 686 maintenance to ensure that each RRset in the zone has a current valid 687 RRSIG RR. The signature validity period of an RRSIG RR is an 688 interval during which the signature for one particular signed RRset 689 can be considered valid, and the signatures of different RRsets in a 690 zone may expire at different times. Re-signing one or more RRsets in 691 a zone will change one or more RRSIG RRs, which will in turn require 692 incrementing the zone's SOA serial number to indicate that a zone 693 change has occurred and re-signing the SOA RRset itself. Thus, 694 re-signing any RRset in a zone may also trigger DNS NOTIFY messages 695 and zone transfer operations. 696 697 9. Name Server Considerations 698 699 A security-aware name server should include the appropriate DNSSEC 700 records (RRSIG, DNSKEY, DS, and NSEC) in all responses to queries 701 from resolvers that have signaled their willingness to receive such 702 records via use of the DO bit in the EDNS header, subject to message 703 size limitations. Because inclusion of these DNSSEC RRs could easily 704 cause UDP message truncation and fallback to TCP, a security-aware 705 name server must also support the EDNS "sender's UDP payload" 706 mechanism. 707 708 709 710 711 712 Arends, et al. Standards Track [Page 13] 713 RFC 4033 DNS Security Introduction and Requirements March 2005 714 715 716 If possible, the private half of each DNSSEC key pair should be kept 717 offline, but this will not be possible for a zone for which DNS 718 dynamic update has been enabled. In the dynamic update case, the 719 primary master server for the zone will have to re-sign the zone when 720 it is updated, so the private key corresponding to the zone signing 721 key will have to be kept online. This is an example of a situation 722 in which the ability to separate the zone's DNSKEY RRset into zone 723 signing key(s) and key signing key(s) may be useful, as the key 724 signing key(s) in such a case can still be kept offline and may have 725 a longer useful lifetime than the zone signing key(s). 726 727 By itself, DNSSEC is not enough to protect the integrity of an entire 728 zone during zone transfer operations, as even a signed zone contains 729 some unsigned, nonauthoritative data if the zone has any children. 730 Therefore, zone maintenance operations will require some additional 731 mechanisms (most likely some form of channel security, such as TSIG, 732 SIG(0), or IPsec). 733
Updates: 1034, 1035, 2136, 2181, 2308, 3225, M. Larson 3007, 3597, 3226 VeriSign
Updates: 1034, 1035, 2136, 2181, 2308, 3225, M. Larson
3007,3597, 3226 VeriSign
734 10. DNS Security Document Family 735 736 The DNSSEC document set can be partitioned into several main groups, 737 under the larger umbrella of the DNS base protocol documents. 738 739 The "DNSSEC protocol document set" refers to the three documents that 740 form the core of the DNS security extensions: 741 742 1. DNS Security Introduction and Requirements (this document) 743 744 2. Resource Records for DNS Security Extensions [RFC4034] 745 746 3. Protocol Modifications for the DNS Security Extensions [RFC4035] 747 748 Additionally, any document that would add to or change the core DNS 749 Security extensions would fall into this category. This includes any 750 future work on the communication between security-aware stub 751 resolvers and upstream security-aware recursive name servers. 752 753 The "Digital Signature Algorithm Specification" document set refers 754 to the group of documents that describe how specific digital 755 signature algorithms should be implemented to fit the DNSSEC resource 756 record format. Each document in this set deals with a specific 757 digital signature algorithm. Please see the appendix on "DNSSEC 758 Algorithm and Digest Types" in [RFC4034] for a list of the algorithms 759 that were defined when this core specification was written. 760 761 The "Transaction Authentication Protocol" document set refers to the 762 group of documents that deal with DNS message authentication, 763 including secret key establishment and verification. Although not 764 765 766 767 Arends, et al. Standards Track [Page 14] 768 RFC 4033 DNS Security Introduction and Requirements March 2005 769 770 771 strictly part of the DNSSEC specification as defined in this set of 772 documents, this group is noted because of its relationship to DNSSEC. 773 774 The final document set, "New Security Uses", refers to documents that 775 seek to use proposed DNS Security extensions for other security 776 related purposes. DNSSEC does not provide any direct security for 777 these new uses but may be used to support them. Documents that fall 778 in this category include those describing the use of DNS in the 779 storage and distribution of certificates ([RFC2538]). 780 781 11. IANA Considerations 782 783 This overview document introduces no new IANA considerations. Please 784 see [RFC4034] for a complete review of the IANA considerations 785 introduced by DNSSEC. 786 787 12. Security Considerations 788 789 This document introduces DNS security extensions and describes the 790 document set that contains the new security records and DNS protocol 791 modifications. The extensions provide data origin authentication and 792 data integrity using digital signatures over resource record sets. 793 This section discusses the limitations of these extensions. 794 795 In order for a security-aware resolver to validate a DNS response, 796 all zones along the path from the trusted starting point to the zone 797 containing the response zones must be signed, and all name servers 798 and resolvers involved in the resolution process must be 799 security-aware, as defined in this document set. A security-aware 800 resolver cannot verify responses originating from an unsigned zone, 801 from a zone not served by a security-aware name server, or for any 802 DNS data that the resolver is only able to obtain through a recursive 803 name server that is not security-aware. If there is a break in the 804 authentication chain such that a security-aware resolver cannot 805 obtain and validate the authentication keys it needs, then the 806 security-aware resolver cannot validate the affected DNS data. 807 808 This document briefly discusses other methods of adding security to a 809 DNS query, such as using a channel secured by IPsec or using a DNS 810 transaction authentication mechanism such as TSIG ([RFC2845]) or 811 SIG(0) ([RFC2931]), but transaction security is not part of DNSSEC 812 per se. 813 814 A non-validating security-aware stub resolver, by definition, does 815 not perform DNSSEC signature validation on its own and thus is 816 vulnerable both to attacks on (and by) the security-aware recursive 817 name servers that perform these checks on its behalf and to attacks 818 on its communication with those security-aware recursive name 819 820 821 822 Arends, et al. Standards Track [Page 15] 823 RFC 4033 DNS Security Introduction and Requirements March 2005 824 825 826 servers. Non-validating security-aware stub resolvers should use 827 some form of channel security to defend against the latter threat. 828 The only known defense against the former threat would be for the 829 security-aware stub resolver to perform its own signature validation, 830 at which point, again by definition, it would no longer be a 831 non-validating security-aware stub resolver. 832 833 DNSSEC does not protect against denial of service attacks. DNSSEC 834 makes DNS vulnerable to a new class of denial of service attacks 835 based on cryptographic operations against security-aware resolvers 836 and security-aware name servers, as an attacker can attempt to use 837 DNSSEC mechanisms to consume a victim's resources. This class of 838 attacks takes at least two forms. An attacker may be able to consume 839 resources in a security-aware resolver's signature validation code by 840 tampering with RRSIG RRs in response messages or by constructing 841 needlessly complex signature chains. An attacker may also be able to 842 consume resources in a security-aware name server that supports DNS 843 dynamic update, by sending a stream of update messages that force the 844 security-aware name server to re-sign some RRsets in the zone more 845 frequently than would otherwise be necessary. 846 847 Due to a deliberate design choice, DNSSEC does not provide 848 confidentiality. 849 850 DNSSEC introduces the ability for a hostile party to enumerate all 851 the names in a zone by following the NSEC chain. NSEC RRs assert 852 which names do not exist in a zone by linking from existing name to 853 existing name along a canonical ordering of all the names within a 854 zone. Thus, an attacker can query these NSEC RRs in sequence to 855 obtain all the names in a zone. Although this is not an attack on 856 the DNS itself, it could allow an attacker to map network hosts or 857 other resources by enumerating the contents of a zone. 858 859 DNSSEC introduces significant additional complexity to the DNS and 860 thus introduces many new opportunities for implementation bugs and 861 misconfigured zones. In particular, enabling DNSSEC signature 862 validation in a resolver may cause entire legitimate zones to become 863 effectively unreachable due to DNSSEC configuration errors or bugs. 864 865 DNSSEC does not protect against tampering with unsigned zone data. 866 Non-authoritative data at zone cuts (glue and NS RRs in the parent 867 zone) are not signed. This does not pose a problem when validating 868 the authentication chain, but it does mean that the non-authoritative 869 data itself is vulnerable to tampering during zone transfer 870 operations. Thus, while DNSSEC can provide data origin 871 authentication and data integrity for RRsets, it cannot do so for 872 zones, and other mechanisms (such as TSIG, SIG(0), or IPsec) must be 873 used to protect zone transfer operations. 874 875 876 877 Arends, et al. Standards Track [Page 16] 878 RFC 4033 DNS Security Introduction and Requirements March 2005 879 880 881 Please see [RFC4034] and [RFC4035] for additional security 882 considerations. 883 884 13. Acknowledgements 885 886 This document was created from the input and ideas of the members of 887 the DNS Extensions Working Group. Although explicitly listing 888 everyone who has contributed during the decade in which DNSSEC has 889 been under development would be impossible, the editors would 890 particularly like to thank the following people for their 891 contributions to and comments on this document set: Jaap Akkerhuis, 892 Mark Andrews, Derek Atkins, Roy Badami, Alan Barrett, Dan Bernstein, 893 David Blacka, Len Budney, Randy Bush, Francis Dupont, Donald 894 Eastlake, Robert Elz, Miek Gieben, Michael Graff, Olafur Gudmundsson, 895 Gilles Guette, Andreas Gustafsson, Jun-ichiro Itojun Hagino, Phillip 896 Hallam-Baker, Bob Halley, Ted Hardie, Walter Howard, Greg Hudson, 897 Christian Huitema, Johan Ihren, Stephen Jacob, Jelte Jansen, Simon 898 Josefsson, Andris Kalnozols, Peter Koch, Olaf Kolkman, Mark Kosters, 899 Suresh Krishnaswamy, Ben Laurie, David Lawrence, Ted Lemon, Ed Lewis, 900 Ted Lindgreen, Josh Littlefield, Rip Loomis, Bill Manning, Russ 901 Mundy, Thomas Narten, Mans Nilsson, Masataka Ohta, Mike Patton, Rob 902 Payne, Jim Reid, Michael Richardson, Erik Rozendaal, Marcos Sanz, 903 Pekka Savola, Jakob Schlyter, Mike StJohns, Paul Vixie, Sam Weiler, 904 Brian Wellington, and Suzanne Woolf. 905 906 No doubt the above list is incomplete. We apologize to anyone we 907 left out. 908 909 14. References 910 911 14.1. Normative References 912 913 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 914 STD 13, RFC 1034, November 1987. 915 916 [RFC1035] Mockapetris, P., "Domain names - implementation and 917 specification", STD 13, RFC 1035, November 1987. 918 919 [RFC2535] Eastlake 3rd, D., "Domain Name System Security 920 Extensions", RFC 2535, March 1999. 921 922 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 923 2671, August 1999. 924 925 [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 926 3225, December 2001. 927 928 929 930 931 932 Arends, et al. Standards Track [Page 17] 933 RFC 4033 DNS Security Introduction and Requirements March 2005 934 935 936 [RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver 937 message size requirements", RFC 3226, December 2001. 938 939 [RFC3445] Massey, D. and S. Rose, "Limiting the Scope of the KEY 940 Resource Record (RR)", RFC 3445, December 2002. 941 942 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 943 Rose, "Resource Records for DNS Security Extensions", RFC 944 4034, March 2005. 945 946 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 947 Rose, "Protocol Modifications for the DNS Security 948 Extensions", RFC 4035, March 2005. 949 950 14.2. Informative References 951 952 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, 953 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 954 RFC 2136, April 1997. 955 956 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 957 Specification", RFC 2181, July 1997. 958 959 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 960 NCACHE)", RFC 2308, March 1998. 961 962 [RFC2538] Eastlake 3rd, D. and O. Gudmundsson, "Storing Certificates 963 in the Domain Name System (DNS)", RFC 2538, March 1999. 964 965 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 966 Wellington, "Secret Key Transaction Authentication for DNS 967 (TSIG)", RFC 2845, May 2000. 968 969 [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures 970 ( SIG(0)s )", RFC 2931, September 2000. 971 972 [RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic 973 Update", RFC 3007, November 2000. 974 975 [RFC3008] Wellington, B., "Domain Name System Security (DNSSEC) 976 Signing Authority", RFC 3008, November 2000. 977 978 [RFC3090] Lewis, E., "DNS Security Extension Clarification on Zone 979 Status", RFC 3090, March 2001. 980 981 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record 982 (RR) Types", RFC 3597, September 2003. 983 984 985 986 987 Arends, et al. Standards Track [Page 18] 988 RFC 4033 DNS Security Introduction and Requirements March 2005 989 990 991 [RFC3655] Wellington, B. and O. Gudmundsson, "Redefinition of DNS 992 Authenticated Data (AD) bit", RFC 3655, November 2003. 993 994 [RFC3658] Gudmundsson, O., "Delegation Signer (DS) Resource Record 995 (RR)", RFC 3658, December 2003. 996 997 [RFC3755] Weiler, S., "Legacy Resolver Compatibility for Delegation 998 Signer (DS)", RFC 3755, May 2004. 999 1000 [RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name 1001 System KEY (DNSKEY) Resource Record (RR) Secure Entry 1002 Point (SEP) Flag", RFC 3757, April 2004. 1003 1004 [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain 1005 Name System (DNS)", RFC 3833, August 2004. 1006 1007 [RFC3845] Schlyter, J., "DNS Security (DNSSEC) NextSECure (NSEC) 1008 RDATA Format", RFC 3845, August 2004. 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 Arends, et al. Standards Track [Page 19] 1043 RFC 4033 DNS Security Introduction and Requirements March 2005 1044 1045 1046 Authors' Addresses 1047 1048 Roy Arends 1049 Telematica Instituut 1050 Brouwerijstraat 1 1051 7523 XC Enschede 1052 NL 1053 1054 EMail: firstname.lastname@example.org 1055 1056 1057 Rob Austein 1058 Internet Systems Consortium 1059 950 Charter Street 1060 Redwood City, CA 94063 1061 USA 1062 1063 EMail: email@example.com 1064 1065 1066 Matt Larson 1067 VeriSign, Inc. 1068 21345 Ridgetop Circle 1069 Dulles, VA 20166-6503 1070 USA 1071 1072 EMail: firstname.lastname@example.org 1073 1074 1075 Dan Massey 1076 Colorado State University 1077 Department of Computer Science 1078 Fort Collins, CO 80523-1873 1079 1080 EMail: email@example.com 1081 1082 1083 Scott Rose 1084 National Institute for Standards and Technology 1085 100 Bureau Drive 1086 Gaithersburg, MD 20899-8920 1087 USA 1088 1089 EMail: firstname.lastname@example.org 1090 1091 1092 1093 1094 1095 1096 1097 Arends, et al. Standards Track [Page 20] 1098 RFC 4033 DNS Security Introduction and Requirements March 2005 1099 1100 1101 Full Copyright Statement 1102 1103 Copyright (C) The Internet Society (2005). 1104 1105 This document is subject to the rights, licenses and restrictions 1106 contained in BCP 78, and except as set forth therein, the authors 1107 retain all their rights. 1108 1109 This document and the information contained herein are provided on an 1110 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1111 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 1112 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 1113 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 1114 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1115 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1116 1117 Intellectual Property 1118 1119 The IETF takes no position regarding the validity or scope of any 1120 Intellectual Property Rights or other rights that might be claimed to 1121 pertain to the implementation or use of the technology described in 1122 this document or the extent to which any license under such rights 1123 might or might not be available; nor does it represent that it has 1124 made any independent effort to identify any such rights. Information 1125 on the procedures with respect to rights in RFC documents can be 1126 found in BCP 78 and BCP 79. 1127 1128 Copies of IPR disclosures made to the IETF Secretariat and any 1129 assurances of licenses to be made available, or the result of an 1130 attempt made to obtain a general license or permission for the use of 1131 such proprietary rights by implementers or users of this 1132 specification can be obtained from the IETF on-line IPR repository at 1133 http://www.ietf.org/ipr. 1134 1135 The IETF invites any interested party to bring to its attention any 1136 copyrights, patents or patent applications, or other proprietary 1137 rights that may cover technology that may be required to implement 1138 this standard. Please address the information to the IETF at ietf- 1139 email@example.com. 1140 1141 Acknowledgement 1142 1143 Funding for the RFC Editor function is currently provided by the 1144 Internet Society. 1145 1146 1147 1148 1149 1150 1151 1152 Arends, et al. Standards Track [Page 21] 1153